Non Compos Mentis

insane, not of sound mind, not sane and cannot be held responsible for his actions

The term non compos mentis comes from Latin, non meaning "not," compos meaning "in control," and mentis, genitive singular of mens, mind, and means not having a sound mind; not sane.It is most typically used in its negative form, non compos mentis, that is, not having control of one's faculties, as in a phrase such as "It was determined that the criminal was non compos mentis" (i.e., insane); one might say the criminal was compos mentis, i.e., the criminal had full knowledge of his/her acts and is competent to stand trial.
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Adjective
1. not of sound mind, memory, or understanding; in law, not competent to go to trial
(synonym) non compos mentis(p), of unsound mind(p)
(similar) insane

Not of sound mind; not having the regular use of reason; hence, also, as a noun, an idiot; a lunatic; one devoid of reason, either by nature or from accident.
  

Not of sound mind (Latin). - (read more on Non compos mentis)